in teenagers – growing taller than expected for the family (with long arms and legs), broad hips, poor muscle tone and slower than usual muscle growth, reduced facial and body hair that starts growing later than usual, a small penis and testicles, and enlarged breasts (gynaecomastia)

These problems can usually be treated if they do occur and testosterone replacement therapy may help reduce the risk of some of them.

Causes of Klinefelter syndrome

Klinefelter syndrome is caused by an additional X chromosome.

This chromosome carries extra copies of genes, which interfere with the development of the testicles and mean they produce less testosterone (male sex hormone) than usual.

The extra genetic information may either be carried in every cell in the body or it may only affect some cells (known as mosaic Klinefelter syndrome).

Klinefelter syndrome isn't directly inherited – the additional X chromosome occurs as a result of either the mother's egg or the father's sperm having the extra X chromosome (an equal chance of this happening in either), so after conception the chromosome pattern is XXY rather than XY.

This change in the egg or sperm seems to happen randomly. If you have a son with the condition, the chances of this happening again are very small.

But the risk of a woman having a son with Klinefelter syndrome may be slightly higher if the mother is over 35 years of age.

Testing for Klinefelter syndrome

See your GP if you have concerns about your son's development or you notice any troubling symptoms of Klinefelter syndrome in yourself or your son.

Klinefelter syndrome isn't necessarily anything serious, but treatment can help reduce some of the symptoms if necessary.

In many cases, it's only detected if a man with the condition undergoes fertility tests.

Your GP may suspect Klinefelter syndrome after a physical examination and may suggest sending off a sample of blood to check reproductive hormone levels.

The diagnosis can be confirmed by checking a sample of blood for the presence of the extra X chromosome.

Treatments for Klinefelter syndrome

There's no cure for Klinefelter syndrome, but some of the problems associated with the condition can be treated if necessary.

Possible treatments include:

testosterone replacement therapy

speech and language therapy during childhood to help with speech development

educational and behavioural support at school to help with any learning difficulties or behaviour problems

Testosterone replacement therapy

TRT involves taking medication containing testosterone. It can be taken in the form of gels or tablets in teenagers, or given as gel or injections in adult men.

TRT may be considered once puberty begins and may help with the development of a deep voice, facial and body hair, an increase in muscle mass, reduction in body fat, and improvement in energy.

You should see a specialist in children's hormones (a paediatric endocrinologist) at this time.

Long-term treatment during adulthood may also help with several other problems associated with Klinefelter syndrome – including osteoporosis, low mood, reduced sex drive, low self-esteem and low energy levels – although it can't reverse infertility.

More information and support

If you or your son has been diagnosed with Klinefelter syndrome, you might find it useful to find out more about it and get in touch with others affected by it.